The present invention is directed to a narrowband laser transmitter comprising a semiconductor laser and an external optical resonator coupled to the semiconductor laser with the outpower power of the transmitter being taken from the external optical resonator. This arrangement for the transmitter enables a wavelength selectivity for the transmitter.
A transmitter of a semiconductor laser and an external optical resonator is proposed in an earlier filed German patent application No. P 36 00 726.9, whose disclosure was incorporated in U.S. patent application Ser. No. 906,503, filed Sept. 12, 1986. In this proposed transmitter, the external optical resonator is fashioned in the form of an optical directional coupler, which is composed of two waveguides extending a slight distance from one another and with a defined coupling length so that crossover of power between the two waveguides will occur in the coupling length. One of the two waveguides is coupled to the semiconductor laser. The laser emissions coupled into these waveguides is supplied to a partially reflective feedback device which conducts a part of the laser emission back into the coupling length. That part of the laser emission that the feedback device allows to pass is conducted to another waveguide of a directional coupler by a loop-shaped optical waveguide. The output power of the transmitter is taken at this other waveguide.
An extremely narrowband, single-mode operating condition is required for future fiber-optic communication systems, particularly with a heterodyne or homodyne reception, can be achieved with such a laser transmitter.
Such a laser transmitter exhibits the advantage that the coupling of the resonator and of the system fiber to the semiconductor laser occurs at only one side of the semiconductor laser, in contrast to other embodiments of the laser transmitter wherein the system fiber is coupled at one side and the resonator is coupled to the semiconductor laser at the other side. As a result of the single-sided coupling, adjustment problems are considerably reduced and the requirement of a high mechanical stability of the fiber of the system and other external resonator can be easily met.